This application is based on application Nos.2000-199132, 2000-365616 filed in Japan, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a surface acoustic wave filter comprising a plurality of surface acoustic wave electrodes arranged on a piezoelectric substrate in a ladder fashion, and more particularly to a surface acoustic wave filter featuring an improved shoulder characteristic curve on a lower frequency of the passband of the filter.
2. Description of the Related Art
As a high-frequency bandpass filter for use in mobile communications devices, the surface acoustic wave filter has been known which comprises a plurality of surface acoustic wave electrodes arranged on the piezoelectric substrate. For instance, there has been disclosed in Japanese Unexamined Patent Publication No.5(1993)-183380 a surface acoustic wave filter wherein a plurality of surface acoustic wave electrodes are arranged on a piezoelectric substrate to define a ladder filter circuit thereon.
FIG. 8 is a schematic circuit diagram for illustrating the surface acoustic wave filter disclosed by the aforesaid prior art.
This surface acoustic wave filter 510 comprises a rectangular piezoelectric substrate 520, on which resonators 530, 540, 550, 560 comprised of surface acoustic wave electrodes are arranged.
As seen in FIG. 8, the resonators 530, 540 are connected in series to form a series arm between an input terminal 570 and an output terminal 580 (hereinafter, the resonators 530, 540 each referred to as xe2x80x9ca series resonatorxe2x80x9d and collectively referred to as xe2x80x9ca series resonator groupxe2x80x9d). On the other hand, the resonators 550, 560 are electrically disposed in parallel between the series arm and a ground electrode 590 (hereinafter, the resonators 550, 560 each referred to as xe2x80x9ca parallel resonatorxe2x80x9d and collectively referred to as xe2x80x9ca parallel resonator groupxe2x80x9d). The series resonators 530, 540 and the parallel resonators 550, 560 are alternatively arranged between the input terminal and the output terminal. The parallel resonators are connected to the ground electrode 590 via inductors 555, 565, respectively.
The series resonator 530 and the parallel resonator 550 are paired to define a one-stage ladder filter. Likewise, the series resonator 540 and the parallel resonator 560 are paired to define a one-stage ladder filter.
The surface acoustic wave filter 510 operates as follows. FIG. 9 is a diagram for explaining a configuration of the surface acoustic wave electrode constituting each of the series or parallel resonators 530-560. FIG. 9 schematically illustrates only an electrode portion of a one-port surface acoustic wave resonator.
In FIG. 9, indicated at 700 is a surface acoustic wave electrode. The electrode 700 has a configuration wherein an interdigital transducer (IDT) 710 is sandwiched between reflectors 720, 730.
The IDT 710 has a configuration wherein a comb electrode 710a having a plurality of electrode fingers 711 and a comb electrode 710b having a plurality of electrode fingers 712 are arranged in such a manner that the electrode fingers 711 are interdigitated with the electrode fingers 712. In this configuration, the comb electrode 710a is connected between an input electrode and an output electrode whereas the comb electrode 710b is connected to the ground electrode, for example.
A surface acoustic wave is excited by a signal inputted to the IDT 710 of the surface acoustic wave electrode 700 thus configured. The excited wave is reflected by the reflectors 720, 730 to form a standing wave which is trapped between the reflectors 720, 730. Thus, the surface acoustic wave electrode 700 operates as a resonator having a high Q value. As is well known, the surface acoustic wave electrode 700 has such impedance characteristics that there exists a pole lowered in impedance by a resonant frequency while there appears a pole increased in impedance by an antiresonant frequency.
The surface acoustic wave filter 510 having the series and parallel resonators 530-560 of such a configuration utilizes the impedance characteristics of the surface acoustic wave electrode 700 for obtaining a passband of a desired bandwidth. More specifically, the resonant frequency of the series resonators 530, 540 and the antiresonant frequency of the parallel resonators 530, 540 are substantially set equal to each other thereby to match the output/input impedance with the characteristic impedance near these frequencies. Thus is established the passband.
Particularly in the ladder filter circuit, the surface acoustic wave electrode 700 has a predetermined impedance characteristic. Therefore, the filter circuit presents a very high impedance near the antiresonant frequency of the series resonators 530, 540 while presenting a very low impedance near the resonant frequency of the parallel resonators 550, 560. Taking advantage of such a characteristic, the ladder filter circuit can obtain a wide filter characteristic ranging from a stop band on a high-frequency side of a passband to a stop band on a low-frequency side of the passband.
As an approach to permit such a ladder filter circuit to achieve an increased attenuation at an attenuation pole, there has been disclosed a resonator provided with an LC circuit comprised of a surface acoustic wave electrode (see, for example, Japanese Unexamined Patent Publication No.9(1997)-232906). Alternatively, a technique has been disclosed wherein the length of a wire connected to an external element is changed to vary the inductance of the wire itself thereby changing the position of an attenuation pole for adjustment of the attenuation thereof (see, for example, Japanese Unexamined Patent Publication No.11(1999)-55067).
Recently, however, the standards for efficient utilization of radio waves have been established for the mobile communications systems. For instance, the US PCS Standards defines the receive band in the range of 1930 to 1990 MHz and the transmission band in the range of 1850 to 1910 MHz so that a gap between the passbands of a transmitting filter and a receiving filter is 20 MHz. Thus, the transmission band is formed adjacent to the receive band. Therefore, the receiving filter is required to ensure a sufficient amount of attenuation and a low insertion loss in the passband while maintaining the wideness of the passband.
This requires a steep characteristic curve on the lower side (left-shoulder side) of the receive band, the curve extending from the stop band to the passband. Unfortunately, neither of the techniques disclosed in Japanese Unexamined Patent Publications Nos.9(1997)-232906 and 11(1999)-55067 is capable of achieving the filter characteristics featuring the increased passband and the steep characteristic curve on the left-shoulder side thereof.
It is an object of the invention to provide a surface acoustic wave filter capable of achieving a sufficiently steep characteristic curve on a lower side of a passband while maintaining the wideness of the passband without use of any specific LC-circuit.
The surface acoustic wave filter according to the invention comprises a series resonator group including a plurality of surface acoustic wave electrodes formed on a surface of a piezoelectric substrate and electrically disposed in series between an input terminal and an output terminal; and a parallel resonator group including a plurality of surface acoustic wave electrodes individually electrically disposed in parallel between an input- or output-terminal of the individual electrodes of the series resonator group and a ground electrode, wherein a resonant frequency formed by a part of the surface acoustic wave electrodes among the surface acoustic electrodes of the parallel resonator group is lower than an antiresonant frequency formed by the other surface acoustic wave electrodes of the prarallel resonator group, and is higher than a resonant frequency formed by the other surface acoustic wave electrodes of the parallel resonator group.
According to the invention, the surface acoustic wave filter is characterized in that a part number of the surface acoustic wave electrodes out of the surface acoustic wave electrodes of the parallel resonator group has a Q-value deteriorative configuration for attaining a degradation of Q value than the Q value of the other surface acoustic wave electrodes of the parallel resonator group.
According to the arrangement of the invention, the filter has such filter characteristics that a new attenuation pole is formed between an attenuation pole on a low frequency side and a low end of a passband. Thus, the filter attains a more steep characteristic curve on the lower side of the passband while maintaining the wideness of the passband, ensuring a satisfactory shoulder characteristic curve.
A specific configuration according to the invention will hereinbelow be described with reference to the accompanying drawings.